Abstract

The nature of light being a subject of intensive research and speculation over the centuries still remains a "dark" issue of modern physics. It has been established that light transfers energy from the source to the receiver by discrete portions, the quanta. However, there is no unified point of view on the nature of the material carrier of the light quantum, that is, the photon. There are several types of photon used in descriptions of the experiments that demonstrate quantum optical effects. The difference in usage of the term "photon" reflects the difference in interpretation of the results of such experiments.

Among quantum optical effects the so-called "essentially quantum effects" that have no classical analogues are worth special mentioning. Such effects cannot be described in the framework of the semi-classical model based on the Maxwell equations, and quantum models are used to describe the effects.